1. Field of the Invention
The present invention relates to a method of transmitting both a useful data signal and an additional data signal on the same channel wherein the additional data signal is first converted into code sequences which are superimposed on the useful data signal with a small amplitude.
2. Description of the Prior Art
In communication networks with non-permanent connections, each connection is provided with an identifier in order to be able to determine whether erroneous connections are occurring as a result of faults in the exchanges. This check is necessary particularly in the case of semi-permanent connections, such as are produced by cross-connectors (network nodes), for example in the synchronous digital hierarchy SDH. These identifiers also need to be transmitted in the case of transparent fully-optical networks in which no access to additional information transported together with the signal as part of the transmission is possible, said information carrying the connection identifier, for example the source identifier, in the existing networks.
German Offenlegungsschrift DE 3 522 130 A discloses a method for transmitting an additional channel in optical transmission systems wherein the additional channel is created without an additional bandwidth of requirement by repeatedly recoding the useful data. However, an appropriate device is then required to recover the useful data.
A further solution is to transmit the identifier on a separate additional channel which is routed parallel to the monitoring connection in all network elements. European patent application EP 0 735 705 A1 discloses a service channel transmitted using a different wavelength. This necessitates an additional frequency requirement. The disadvantage of this method is that it is not possible to prevent the additional channel, transmitting the identifier, and the useful signal from being allocated separately or incorrectly on account of a fault in a network element; for example, of the cross-connector. This means, of course, that reliable detection of an erroneous connection is no longer possible. It is possible, however, for an erroneous connection to be simulated.
The problem, therefore, is that reliable association of the useful data signal and the additional data signal is necessary with optical connections. To do this, the transmission of useful data, described in EP 0 735 705, needs to be modified.
Accordingly, pursuant to the teachings of the present invention, both the useful data and the additional data are transmitted on the same channel wherein association is always guaranteed. Transmission on the same channel is enabled as a result of the additional data signal first being converted into code sequences which are then superimposed on the useful data signal with a small amplitude.
The use of a code signal converted to a higher frequency band can be advantageous because it means that extremely low-frequency spectral components are avoided. Problems in the lower frequency range of fiber amplifiers are avoided with a carried additional signal.
In all cases, superimposing the additional data signal on the useful data signal only results in the signal-to-noise ratio being slightly worsened; the additional data signal having the effect of slightly increasing the channel noise. Even if the signal-to-noise ratio is generally poor as a result of transmission, the additional data signal still can be detected. If each bit of the additional data signal is allocated a plurality of successive code sequences, the detectability is improved further.
It is advantageous if the additional data signal is a binary signal as this has the largest signal-to-noise ratio wherein the two logic states can be transmitted as inverted code sequences.
It is also prefer that superimposition takes place only during the on-states of the useful data signal, if this signal is transmitted by way of keyed modulation. This is easier to implement [lacuna] in terms of circuitry.
The method can, in principle, be used with all types of modulation; for example, with phase modulation.
The method is particularly suitable for transmitting identifiers or addresses in optical transmission systems. The additional data signal can be checked without accessing the useful data signal, so that it is constantly possible to check the correctness of a connection. A particular identifier, i.e. a particular 1,0 sequence, can be repeatedly transmitted without interruption so as to minimize the detection time. However, transmission using time-division multiplexing with further additional data is also possible. Furthermore, the method also can be used, of course to transmit any desired additional signals.
Additional features and advantages of the present invention are described in, and will be apparent from, the Detailed Description of the Preferred Embodiments and the Drawing.